1. Field of the Invention
The disclosure described herein generally relates to drill bits for use in drilling operations in subterranean formations. More particularly, the disclosure relates to hybrid bits, and the pin angle of rolling cutters in the hybrid bit in conjunction with fixed blades of the hybrid bit.
2. Description of the Related Art
Drill bits are frequently used in the oil and gas exploration and the recovery industry to drill well bores (also referred to as “boreholes”) in subterranean earth formations. There are two common classifications of drill bits used in drilling well bores that are known in the art as “fixed blade” drill bits and “roller cone” drill bits. Fixed blade drill bits include polycrystalline diamond compact (PDC) and other drag-type drill bits. These drill bits typically include a bit body having an externally threaded connection at one end for connection to a drill string, and a plurality of cutting blades extending from the opposite end of the bit body. The cutting blades form the cutting surface of the drill bit. Often, a plurality of cutting elements, such as PDC cutters or other materials, which are hard and strong enough to deform and/or cut through earth formations, are attached to or inserted into the blades of the bit, extending from the bit and forming the cutting profile of the bit. This plurality of cutting elements is used to cut through the subterranean formation during drilling operations when the drill bit is rotated by a motor or other rotational input device.
The other type of earth boring drill bit, referred to as a roller cone bit, typically includes a bit body with an externally threaded connection at one end, and a plurality of roller cones (typically three) attached at an offset angle to the other end of the drill bit. These roller cones are able to rotate about bearings, and rotate individually with respect to the bit body.
An exemplary roller cone bit and cutting roller cone are illustrated in FIGS. 1A and 1B and described in U.S. Pat. No. 6,601,661, incorporated herein by reference. The roller cone bit 10 includes a bit body 12 having a longitudinal centerline 8 and having a threaded pin-type connector 14 at its upper end known as a “shank” for coupling the bit body 12 with the lower end of a drill string (not shown). The bit body 12 has generally three downwardly depending legs (two shown as legs 16, 18) with a lubricant compensator 20 provided for each. Nozzles 22 (one shown) are positioned between each of the adjacent legs to dispense drilling fluid during drilling. The drilling fluid is pumped down through the drill string and into a cavity (not shown) in the bit body 12. A roller cone is secured to the lower end of each of the three legs. The three roller cones 24, 25, and 26 are visible in FIG. 1 secured in a rolling relation to the lower ends of the legs of bit body 12.
The roller cone 24 has a cutter body 32 that is typically formed of suitably hardened steel. The cutter body 32 is substantially cone-shaped. A plurality of primary cutting elements 34, 36, 38 extend from the cutter body 32. When the cutter body 32 is rotated upon the spindle 28, the primary cutting elements engage earth within a borehole and crush it. The plurality of cutting elements may be one or a combination of milled steel teeth (called steel-tooth bits), tungsten carbide (or other hard-material) inserts (called insert bits), or a number of other formed and/or shaped cutting elements that are formed of materials having a hardness and strength suitable enough to allow for the deformation and/or cutting through of subterranean formations. In some instances, a hard facing material is applied to the exterior of the cutting elements and/or other portions of the roller cone drill bit, to reduce the wear on the bit during operation and extend its useful working life.
The roller cone 26 is rotatably retained by bearings 27 on a spindle 28 having a spindle base 29 that joins the roller cone leg 18. The spindle 28 has an axis of rotation 6 that is at some angle “α”, known as a “pin angle”. The pin angle is measured between the spindle axis of rotation 6 and a datum plane 7. The datum plane 7 is formed orthogonal to the longitudinal centerline 8 of the bit. The datum plane 7 intersects the spindle axis of rotation 6 near the spindle base 29, as illustrated in FIG. 1A. The plane 7 can be represented pictorially as a horizontal plane when the bit centerline 8 is vertical with the shank oriented upright and the cutters facing downwardly, as seen in FIG. 1A. The spindle base 29 is the region of the junction between the spindle 28 and the roller cone leg 18, and generally is located proximate to the intersection of the rear face 30 of the roller cone 26 and the spindle axis of rotation 6. The pin angle “α” is measured in a plane 9 that is orthogonal to the plane 7 and contains the spindle axis of rotation 6. The pin angle is measured in a counterclockwise direction from the datum plane 7 to the spindle axis of rotation 6 starting at the intersection of the plane 7 with the bit centerline 8, when viewed with the spindle 28 oriented to the right of a vertical centerline 8. The pin angle “α”, as illustrated in FIG. 1A, measures approximately 33 degrees. It should be noted that the axis of rotation 6 may not intersect the bit longitudinal centerline 8, if the bit has offset.
The pin angle from the plane 7 to the axis of rotation 6 of the roller cone can be generally from 33 degrees to 39 degrees, with 33 degrees to 36 degrees being customary. The pin angle is critical to establishing the intermeshing of the roller cones and their cutting elements. Further, the pin angle significantly affects the load on the rolling cone and its spindle for radial and thrust loads. Generally, a smaller pin angle, such as 33 degrees, will be used for softer cutting formations, where a smaller pin angle allows the cutting elements to have a greater projection outwardly for more engagement with the formation. A larger pin angle, such as 36 degrees, will generally be used for harder cutting formations, where the cutting elements have less projection into the formation. The pin angle further affects and is affected by roller cone bearing size, the number of rolling cones, projection length and shape of the cutting elements on the rolling cone, leg strength, roller cone diameter, shape of the rolling cone, and other factors. The pin angle is empirically picked and has been standardized between the above referenced angles of 33 degrees to 39 degrees with 33 degrees to 36 degrees being the most common. A small change can yield significant differences in the roller cone performance, and some pin angles are determined in increments of less than 1 degree.
These general classes of earth boring bits have limitations, particularly with the bit life and the types of subterranean formations through which they can drill. Fixed blade bits using PDC cutting elements, and therefore known as “PDC bits”, usually can be used with success in soft to medium-hard formations. Hard and/or abrasive formations are generally considered more challenging for PDC bits in that their use in such formations results in excessive wear and shortened working life. For example, mudstone and siltstone have been drilled well; however, sandstones, particularly if coarse-grained and cemented, are very difficult to drill economically and are highly destructive to fixed blade drill bits. [See, for example, Feenstra, R., et al., “Status of Polycrystalline-Diamond-Compact Bits: Part 1—Development” and “Part 2—Applications”, Journal of Petroleum Technology, Vol. 40 (7), pp. 675-684 and 817-856 (1988).] Success is fully dependent on a good match between the bit, the formation to be drilled, and the operating conditions. Experience has shown that for fixed blade bits such as PDC bits, the type of mud, the bit hydraulics, and bit design may affect bit performance.
Repeated experience shows that a preferred practice is to develop the best bit design for a particular field rather than to select one from a range. Increased aggressiveness in earth-boring bits is not always desirable, because of the increase torque requirements that are generally associated with it. The ability to design and/or tailor a bit to a particular subterranean operation or application can be an invaluable tool for the bit designer. Thus, in recent years, attempts have been made to develop earth-boring drill bits that use a combination of one or more rolling cutters and one or more fixed blades having PDC or similarly abrasive cutting elements formed or bonded thereon. Some of these combination type bits are referred to as “hybrid drill bits”.
One previously described hybrid drill bit is disclosed in U.S. Pat. No. 4,343,371, “wherein a pair of opposing extended nozzle drag bit legs are positioned with a pair of opposed tungsten carbide roller cones. The extended nozzle face nearest the hole bottom has a multiplicity of diamond inserts mounted therein. The diamond inserts are strategically positioned to remove the ridges between the kerf rows in the hole bottom formed by the inserts in the roller cones. A cross section of the pilot pin and journal is not shown in the above patent, but is typically the same as a roller cone bit.
The typical practice heretofore has been to combine the fixed blades with a modified roller cone (herein a “rolling cutter”) using the same pin angles of a roller cone drill bit. The additional space used by the fixed blades requires that the size of the roller cones be reduced to fit with the blades. The size of the roller cones on a hybrid bit will generally be smaller than the cones on a roller cone bit of the same diameter. The reduced cone size may result in fewer cutting elements, smaller diameter cutting elements, reduced bearing diameter and length, and other compromises. Some unique drill bits vary from the standard pin angles, but appear to be limited to single fixed blade and single rolling cutters. These somewhat rare and special purpose drill bits are not constrained by the interrelationships of multiple fixed blades and multiple rolling cutters. Thus, the teachings of such unique drill bits are not transferable to a drill bit with multiple fixed blades and multiple rolling cutters.
There remains a need for an improved hybrid bit that can better optimize the interrelationships between the fixed blades and rolling cutters.